When working in heat, most workers wear clothing (e.g., long-sleeved shirt and trousers. However, most heat stress research has used semi-nude subjects. Workers also tend to go in and out of heat all day (intermittent exposure) while most research has continuous heat exposure for 1 or 2 hours. Most workers are heat acclimatized; much research uses non-acclimatized subjects. Our project has five phases: clothing worn in the field, insulation values, physiological responses, model validation and intermittent heat stress guidelines. In Phase 1, we will determine, for at least 25 organizations, what articles are actually worn in heat stress and recovery. In Phase 2, we will determine, using our electrically heated copper mannequin, insulation values (clo) of typical ensembles from Phase 1. In Phase 3, we will measure physiological responses of 12 acclimatized subjects at 4 temperature levels, 3 work-rest cycles, and 3 clothing levels and 2 recovery levels. Each session will last 10.5 hours (pretest, heat, post-test sessions with time for subjects to have coffee and lunch breaks in controlled environments). Heart rate, skin temperature, rectal temperature and clothing surface temperatures will be recorded automatically on tape every five minutes. Weight loss will be recorded every 60 min. In Phase 4, from our previous data and from Phase 3, we will validate our biophysical 25 node human thermoregulatory model for clothed exercising individuals exposed to intermittent heat stress. In Phase 5, from the experimental data and computer simulations using the model, guidelines will be developed giving permitted exposure to intermittent heat stress for clothed workers.